1. Field of the Invention
The present invention relates to a semiconductor device and a method for manufacturing the semiconductor device.
2. Background of the Related Art
A method has been known which electrically connects (bonds) a top side electrode of a semiconductor element and a circuit layer with a wire, such as an aluminum (Al) wire, in a semiconductor device with a module structure that is used in, for example, a general-purpose inverter, a wind power generation system, a photovoltaic generation system, and an electric railroad. The top side electrode is, for example, an aluminum-silicon (Si) electrode or an Al—Si-copper (Cu) electrode and the circuit layer is made of a conductor, such as Cu. In recent years, wire bonding using a Cu wire instead of the Al wire has been examined. The structure of a semiconductor device with a module structure according to the related art will be described below.
FIG. 10 is a cross-sectional view illustrating the structure of the semiconductor device with the module structure according to the related art. FIG. 11 is a cross-sectional view schematically illustrating the structure of a semiconductor element illustrated in FIG. 10. As illustrated in FIGS. 10 and 11, the semiconductor device with the module structure according to the related art includes a semiconductor element 101, an insulated substrate 102, such as a ceramics insulated substrate (direct copper bonding (DCB) substrate), a Cu base 106, and an Al wire 107. In the insulated substrate 102, a circuit layer 104 made of Cu is provided on a front surface of an insulated layer 103 and a copper layer 105 is provided on a rear surface of the insulated layer 103. The semiconductor element 101 includes a top side electrode 112 which is provided on a front surface of a semiconductor die 111 and a bottom side electrode 113 which is provided on a rear surface of the semiconductor die 111.
The bottom side electrode 113 of the semiconductor element 101 is bonded to the circuit layer 104 through a solder layer 101a. The top side electrode 112 of the semiconductor element 101 is electrically connected to the circuit layer 104 by wire bonding using, for example, an Al wire 107. The Al wire 107 is bonded using ultrasonic vibration. Conditions, such as heat, ultrasonic vibration, and welding pressure, for the diameter of the Al wire 107 are optimized to achieve good bonding, without any connection failure. The front surface of the Cu base 106 is bonded to the copper layer 105 through a solder layer (not illustrated).
As the semiconductor device with the module structure, a device has been proposed which includes an aluminum wire that connects an element and an external electrode, has an average crystal grain size of 50 μm or more, and contains 0.02 wt % to 1 wt % of Fe in order to increase a recrystallization temperature to 150° C. or higher such that recrystallization does not occur when the device is turned on, for example, see JP 8-8288 A (Patent Document 1).
As another device, a device has been proposed in which an aluminum wire bonding portion that is bonded to an electrode pad on a semiconductor element by a wire has a uniform crystal grain size, for example, see JP 7-135234 A (Patent Document 2). As still another device, a device has been proposed which contains 4.5 wt % to 6.5 wt % of copper, 0.1 w % to 1.0 w % of zirconium (Zr), and the balance of aluminum, for example, see JP 61-163235 A (Patent Document 3).
As yet another device, a device has been proposed which includes electrodes that are provided on the front and rear surfaces of a semiconductor element, a circuit layer that is provided on an insulated substrate and is bonded to the bottom side electrode, a metal protective film that is formed on the upper surface of the top side electrode, and a wire that is electrically bonded to the top side electrode through the metal protective film by thermo-compression bonding or ultrasonic vibration, for example, see JP 2009-76703 A (Patent Document 4).
However, in recent years, there has been an increasing demand for a technique which increases the amount of current, achieves a high-temperature operation, or increases reliability with the expansion of the purpose of use. In order to meet the demand, it is indispensable to improve power cycle resistance (lifetime). For example, whenever a new technique is proposed, the size of the semiconductor device is reduced and a mounting area is reduced. Therefore, it is difficult to increase the number of Al wires in order to increase the amount of current and there is a concern that the Al wire will be melted and broken due to the generation of heat from the wire when a large amount of current flows to each Al wire.
Factors for determining the power cycle resistance are the maximum value Tjmax of the bonding temperature of the semiconductor element and a variation ΔTj in the bonding temperature of the semiconductor element due to the intermittent flow of a current. In particular, components of the semiconductor device have different linear expansion coefficients and stress corresponding to the difference between the linear expansion coefficients is applied to each component. As the maximum value Tjmax of the bonding temperature of the semiconductor element increases, stress corresponding to the difference between the linear expansion coefficients of the components increases. As a result, there is a concern that the element will be broken. For this reason, it is necessary to improve the power cycle resistance during a high-temperature operation and to achieve a high-temperature operation and high reliability at the same time.
In the semiconductor device according to the related art, the crystal grains in the vicinity of the bonding interface of the Al wire are changed to fine grains by the wire bonding and the bonding strength of the Al wire is improved. Thereafter, the crystal grains in the vicinity of the bonding interface of the Al wire and in the top side electrode are coarsened and softened by a thermal history obtained by a manufacturing process after the wire bonding or the high-temperature operation (for example, about 175° C.) when the semiconductor element is turned on and generates heat. As a result, a crack (breaking) is likely to occur in the vicinity of the bonding interface between the Al wire and the top side electrode. The inventors performed the power cycle test and found that cracks occurred in the Al wire and extended to the inside of the wire with an increase in the number of power cycles, and the Al wire was finally broken and detached, which resulted in the breakdown of the element.
The invention has been made in order to solve the above-mentioned problems of the related art and an object of the invention is to provide a semiconductor device and a method for manufacturing a semiconductor device which can improve power cycle resistance when a large amount of current flows and during a high-temperature operation.